Wireless networks are evolving from circuit-switched networks (e.g., global system for mobile communications (GSM), IS-136, IS-95) to packet-switched networks (e.g., wireless local area network (WLAN), Universal Mobile Telephone System (UMTS), cdma2000) configured to support multimedia applications to mobile end-users over Internet protocol (IP). General Packet Radio Service (GPRS), which is an evolution of GSM, may support packet data (e.g., web browsing, email) in a cellular environment. Further evolution of GPRS, often referred to as the UMTS, is expected to support real-time multimedia over IP (e.g., voice over IP (VoIP), video over IP, streaming media) in a cellular environment. In addition, the Third Generation Partnership Project (3GPP) has specified the IP Multimedia Subsystem (IMS) in UMTS to accomplish the control and service functions of wireless IP multimedia. In this regard, the 3rd Generation Partnership Project (3GPP) has adopted Session Initiation Protocol (SIP) as the signaling protocol in IMS. At the same time, in the cdma2000 world, the 3GPP2 has been developing the IMS, formerly referred to as the IP Multimedia Domain (MMD), to implement the control and service functions of wireless IP multimedia. The 3GPP2 has also adopted SIP into the IMS specification.
And although the IMS provides a number of enhanced features in the provision of IP-based telephony and multimedia services, it is generally desirable to improve upon existing techniques.